The Applicant has previously described a method of enabling users to access information from a computer system via a printed substrate e.g. paper. The substrate has a coding pattern printed thereon, which is read by an optical sensing device when the user interacts with the substrate using the sensing device. A computer receives interaction data from the sensing device and uses this data to determine what action is being requested by the user. For example, a user may make handwritten input onto a form or make a selection gesture around a printed item. This input is interpreted by the computer system with reference to a page description corresponding to the printed substrate.
It would desirable to improve the coding pattern on the substrate so as to maximize the data capacity of the coding pattern and minimize the overall visibility of the coding pattern on the substrate. The coding pattern typically comprises data symbols (encoding ‘useful’ information, such as an identity and/or a location) and as well as other symbols, which allow the data symbols to be decoded. It would be desirable to minimize the amount of data encoded in these other symbols so as to make maximum use of the data symbols. By minimizing the space occupied by these other symbols, there is more space available in the coding pattern for data symbols which encode useful information.
It would be further desirable to distribute coordinate data symbols so as to maximize the error-correcting capacity of the code. In particular, it would be desirable to provide maximum robustness even when, for example, the coding pattern is scratched with horizontal or vertical lines (e.g. if the coding pattern is co-printed with form field boxes containing horizontal and vertical lines, which potentially affect the readability of the coding pattern).